1. FIELD OF THE INVENTION
The invention relates to reaction injection molding of polymeric materials and more particularly to a system for supplying a mixing head of such a system with a plurality of liquid components which are maintained in separate circulatory paths until mixed together at the mixing head and injection into a suitable mold. The invention pertains specifically to the design of the circulatory system wherein a portion of the recycling conduits of the several liquid components are used intermittently as supply conduits operating in parallel flow with the normal supply conduits of the system.
2. Description of the Prior Art
An example, for which the invention is particularly suitable, is the so-called reaction foam casting method. In the case of this method two or more fluid plastics components are supplied to a mixing head, where they are mixed and injected into the casting mold. In order to supply the plastics components only during the times of injection to the mixing head, the usually adopted practice up till now has been that of providing one inlet valve at the mixing head for each respective component, and this valve is opened during the injection times and is closed in the pauses or intervals. The fluid plastics component is passed by a pump from a feed container via a supply line to this inlet valve. In order to make possible circulation of the plastics component during the pauses, the inlet valve is furthermore connected with a return line, which returns to this feed container. During the pauses the inlet valve closes the opening, connecting these lines with the space to be charged, in this case the mixing chamber of the mixing head, and only the connection between the supply and return lines remains open so that the plastics material in this line system can circulate back to the feed container. During the injection periods the inlet valve is opened so that a connection between the supply and return lines or ducts on the one hand and the mixing chamber to be charged with the plastics material is afforded. Furthermore a stop cock located in the return line is closed which interrupts the circulation so that by means of the conveying pump a pressure can be built up in the supply line and this pressure forces the plastics material through the inlet valve into the mixing chamber of the mixing head.
The disadvantage of this prior art supply device resides in that the whole flow of plastics material must be propelled also in intervals between the individual injection periods via the inlet valve of the mixing head. Owing to the substantial viscosity of the plastics material this leads to a substantial heating of the material which makes necessary the provision of additional cooling means in the line or duct system. In order to keep down the heating effect as far as possible the line cross-sections, the cross-sections of the supply lines to the mixing head and the passage cross-sections in the mixing head must be made large, something which makes all dimensions as a whole large. The expense of the equipment is thus not only substantial, but the large duct lines make the mixing head difficult to handle.